I have owned my 87 Wolfsburg since it was new.  My mileage has never strayed from 19-22 mpg.  I did get the best mileage (22 mpg) on my trip to Montana in the early 90s (I filled up somewhere near Yakima, then again in Spokane on the way to Missoula).  Usually climbing the grades to Reno I get 19mpg when I fill up when I get to Reno, then 20-21 on the way back from Reno (mostly downhill from Donner Summit all the way to Roseville).  I think Donner Summit is about 7200'; Sonora Pass is the next highest (over 9,000); then Tioga Pass (also over 9,000, a little higher than Sonora).  I've driven over most of the Sierra passes numerous times and have not strayed from the 19-22mpg range.

Donna, 1987 Wolfsburg, 1982 Diesel